1. Field of the Invention
The present invention relates to an insert boring tool comprising a tool body and radially outward and inward cutting inserts provided at a forward end of the tool body.
2. Prior Art
A conventional boring tool of the kind referred to above will be described with reference to FIGS. 1 through 4 of the accompanying drawings. As shown in FIG. 1, the insert boring tool has a tool body 11 which is provided with two chip discharging flutes or grooves 12 and 12. These chip discharging flutes 12 and 12 extend axially from a forward end of the tool body 11 toward a rear portion thereof which serves as a shank 13. A radially outward cutting insert 14 generally in the form of a regular triangle as viewed in plan is detachably secured at a forward end of one of the chip discharging flutes 12 and, likewise, a radially inward cutting insert 15 in the form of a regular triangle as viewed in plan is detachably secured at a forward end of the other chip discharging flute 12. These radially outward and inward inserts 14 and 15 are so arranged as to have their respective loci of rotation shown in FIG. 2, in a plane containing an axis X of the tool body 11. Specifically, the radially outward and inward inserts 14 and 15 are arranged such that an end cutting edge 16 of the radially outward insert 14 is intersected at a point of intersection P with an end cutting edge 17 of the radially inward insert 15. Further, the radially outward and inward inserts 14 and 15 are positioned such that, assuming that a radial distance between the point of intersection P and a radially outermost end of the end cutting edge 16 of the radially outward insert 14 is A, and a radial distance between the point of intersection P and a radially innermost end of the end cutting edge 17 of the radially inward insert 15 is B, A/B becomes equal to 48/52 to 1/1.
The insert boring tool as describe above is fed forwardly while being rotated about the axis X, to cause the radially outward and inward inserts 14 nd 15 to perform a boring.
Since the arrangement of the above-described insert boring tool is such that the radially outward and inward 14 and 15 overlap each other in their respective loci of rotation, only portions of their respective end cutting edges 16 and 17, that is, only regions A and B participate in the cutting. For this reason, the principal force distribution during boring takes the form shown in FIG. 3 in which a principal force E acting on the radially outward insert 14 and a principle force F acting on the radially inward insert 15 have the relationship of E/F.apprxeq.3/1. Further, a back force G acting on the radially outward insert 14 and a back force H acting on the radially inward insert 15 have the relation of G.apprxeq.H. As a result, as shown in FIG. 4, the relationship between a resultant cutting force P of the principal force E and the back force G applied to the radially outward insert 14 and a resultant cutting force Q of the principal force F and the back force H applied to the radially inward insert 15 becomes P.apprxeq.3.1Q, causing an extreme unbalance therebetween. This makes the cutting unstable, and deteriorates boring accuracy, finished surface roughness and bore straightness. Specifically, since the tool body 11 is liable to lean in the direction of the resultant cutting force P, Q, the hole formed by the tool may have a diameter smaller than the outer diameter of the tool at its deep portion, thereby lowering a boring depth limit.